According to the Topix FAQ (frequently asked questions), you can't.
I am providing a link below.
See the question, "Can I delete my own posts?" and the question, "Can I get posts removed from the site?"
As I stated above, the answer to the first question is 'no.' The answer to the second question is 'yes.' Instructions are provided for removing objectionable material such as you describe from the site.
I hope this helps.